Cost is one, and sometimes a dominating, factor considered when designing silicon devices for consumer and automotive electronics. There are well known constraints imposed on the circuit designer by process technology and/or masks in order to accomplish cost reduction. These constraints often necessitate the use of lower voltage components in situations where it is known or anticipated that the circuit will be exposed to higher voltage conditions. For example, it is not uncommon for a given process technology and/or masks to produce components having a certain design maximum voltage (for example, 45V). Cost considerations may require the use of such process technology and/or masks, and thus corresponding devices having a relatively lower maximum voltage, when designing and producing a circuit for a higher voltage (for example, 60V) application. When designing a circuit using components whose maximum voltage is less than the circuit application voltage, special care must be taken to ensure that no individual device will be operated in the higher voltage application with a voltage that exceeds the lower maximum voltage rating. For example, the designer may utilize a number of protective zener diodes to protect the Vds and Vgs of included MOSFET devices. This solution adds to the complexity of the circuit design, and the additional protective components undesirably increase circuit die size.
There is a need in the art for a circuit design technique to address the complexity and die size problems noted above, while permitting the circuit designer to use components whose maximum voltage rating is less than the circuit voltage.
It is known in the art to provide cascode devices in high voltage circuits. The gate of the cascode device is connected to a fixed reference voltage. A problem with this configuration is that the cascode device, with a gate connected to the supply rail, will preclude operation of the circuit support a swing in the voltage at the output node to the supply rail voltage when the switching transistor is activated.
There is a need in the art for a circuit design technique to address the foregoing problem.